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Development of Multilayer Organic Modules for Hermetic Packagingof RF MEMS Circuits

Morgan Chen', Anh-Vu Pham', Chris Kapusta2, Joe Iannotti2, William Kornrumpf2, Nicole Evers2, JohnMaciel3, and Nafiz Karabudak4

Microwave Microsystems Laboratory, Department of Electrical and Computer Engineering, University ofCalifornia at Davis, CA 95616 USA

2General Electric Global Research Center, Niskayuna, NY 12309 USA

3Radant MEMS, Stow, MA 01775 USA

4Lockheed Martin Commercial Space Systems, Newtown, PA 18940 USA

Abstract - We present the design and development of a multi-layer organic module that can integrate Micro-Electro-Mechanical Systems (MEMS) into a System-in-a-Package (SiP).A cavity formed in Liquid Crystal Polymer (LCP) has beenlaminated, at low temperature, onto a MEMS silicon switch tocreate a hermetically sealed package. Multi-layer organicdielectrics can be integrated on top of LCP films to form a 3-DSiP module. The entire SiP hermetically sealed package has atotal insertion loss of 40.1 dB at X-band. The package also passesMethod 1014, MIL-STD-883 gross leak and fine leak hermeticitytests. We have demonstrated a 2-bit RF MEMS TTD (True-Time Delay) switched line phase shifter in this multi-layerorganic module. The phase shifter achieves an average insertionloss of 1.8 dB/bit, with less than 3° phase shift variation.

Index Terms - Liquid Crystal Polymer, Hermeticity, PhaseShifter.

I. INTRODUCTION

MEMS circuitry has been identified as an emergingtechnology to replace bulky RF/microwave systems with theirminiaturized versions. MEMS devices may offer enhancedperformance at lower cost and bulk. A number of solutionsare available for packaging MEMS switches. Severaltechniques used by industry to package MEMS devicesinclude epoxy seals, glass frit, glass-to-glass anodic bonding,and gold-to-gold bonding [1-3]. These techniques face twomain problems. First, organic materials outgas inside theMEMS cavity during bonding processes due to wettingcompounds in the glass, gold, or epoxy layers. Thiscontamination causes a serious detrimental effect on thereliability lifetime of the MEMS switches. Second, to achievea good seal, most bonding processes utilize high temperatures(300-4000C) that can degrade MEMS structures [3].Furthermore, available hermetic packages and ceramic/glassfeed-throughs have significant parasitic losses at microwavefrequencies, can be expensive, and add significant weight to asystem. Packaging MEMS switches into an SiP, in which

compact, multi-layer substrates house active and passivecomponents present even more challenges. Although multi-layer chip-on-flex modules using KaptonTM films are a proventechnology for high-density module integration, [4, 5]KaptonTm had been found to be incompatible with RF MEMSswitch packaging due to its high moisture absorption,outgassing characteristics, and requirements that highoutgassing epoxies are used for lamination.

In this paper, we present the development of a hermeticallysealed package for RF MEMS switches in chip-on-flexmodules using Liquid Crystalline Polymer. We havedeveloped a lamination process to adhere LCP onto silicon toform a hermetically sealed enclosure for MEMS [6]. Usingmulti-layer flex and laser drilled vias, the first levelinterconnect parasitics and losses are found to be negligible atX-band. Microwave measurements demonstrate that the LCP-package has -0.1 dB insertion loss and maintains the returnloss of a switch to greater than 25 dB. The LCP MEMSpackage passes the E595 outgassing test and Method 1014,MIL-STD-883 gross leak and fine leak hermeticity tests.Using this technology, a multi-layer organic module isdeveloped that can integrate MEMS in an SiP platform. A 2-bit RF MEMS True-Time Delay (TTD) switched line phaseshifter is fabricated and tested to demonstrate the multi-layerorganic module with hermetic capabilities.

11. PACKAGING THE BARFE RF MEMS SWITCHES

Figure 1 demonstrates the first level packaging enclosurethat provides hermeticity to an RF MEMS switch. Firstly,copper is sputtered and electroplated onto thin-film LCP.Secondly, the LCP film is laser ablated up to the coppercladding to form 1) the fiduccial markings for alignment and2) the cavity where the MEMS switch will be hermeticallysealed. If looking into the opening made by the laser at thisprocessing step, one would see a cavity with a copper surface

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base surrounded by LCP sidewalls. Thirdly, the cavity isaligned and laminated to the silicon substrate containing theMEMS switch. Lastly, lithography and via formation stepsare performed, and the first level interconnect process iscompleted. In this proposed integration scheme, MEMSdevices are protected by LCP along sidewalls, silicon frombelow, and copper from above. The low-temperaturelamination process has alignment of features accurate to 1 ptmand LCP displacement reflow to only 5 ptm at the criticalfeatures. Figure 2 demonstrates a packaged RF MEMS switchin hermetic enclosure with ultra-low parasitic interconnect.

Copper Switch Covering==;,

\

Copper RF Trace/

Fig. 1. Cross-section of the packaged

DC Bias

III. SYSTEM-ON-PACKAGE IMPLEMENTATION

Subsequent LCP films or Kapton films can be integrated onthe first level interconnect to form high density interconnects.Using this technology, we have designed and implemented a2-bit phase shifter, in which MEMS switches are packagedinside LCP and delay lines are designed on low loss polymerfilms. Figure 3 demonstrates the multi-layer system in apackage that has a hermetic cavity for housing an RF MEMSswitch. MEMS switches are packaged as discussed in section11 of this paper and are integrated on 2-mil thin DupontKapton polyimide. Firstly, bare RF MEMS switches arelaminated to LCP. Secondly, first level via interconnects areformed through LCP to contact the MEMS part. Thirdly, LCPis metallized and patterned. Fourthly, the stack-up islaminated to the Kapton substrate with an epoxy. Fifthly,second layer via connections are formed through Kapton tocontact metal pads on the LCP layer. Finally, Kapton ismetallized and patterned for transmission line delay structures.Microstrip lines are designed in Agilent ADS LineCalc with 4pm copper on 50 pm Dupont Kapton polyimide. By usingboth LCP and Kapton dielectric substrates, the SiP integrationbenefits by providing excellent hermetic environments that arecompatible with well-known, reliable Kapton thin-filmprocessing.

RF path

Overview of Kapton/LCP stackupSTD COF HDI processing

RF MEMS Switch

(a) (b)Fig. 2 (a) Top-down and (b) 3D perspective photographs ofprototypes of the packaged MEMS switch in LCP Fig. 3 Cross section of the multi-layer organic module that integrates

a hermetic environment

We have evaluated hermetic properties of packaged RFMEMS switches at Six Sigma Services [7]. LCP packageshave undergone and passed Mil-Std 883 Method 1014 grossand fine leak hermetic testing. Packages have also survived72 hours of continuous moisture testing at 98 °C with noelectrical degradation. Furthermore, low outgassing propertieshave been characterized by ASTM E595 testing and resultsfrom this test showed that the LCP used in the enclosureachieved an average of 0.038% Total Mass Loss (TML),0.0040O Collected Volatile Condensable Materials (CVCM),and 0.0310% Water Vapor Regain (WVR). This is better thanthe limits on the historical standards for Space qualification,which specify maximum levels of 1% TML and 0.1% CVCMlimits. In our process, the LCP is also tested to show excellentdimensional stability and adhesion to copper, silicon, andother LCP laminates in excess of 3 lbs/in for each case. Anycontamination to the MEMS switch could result in stiction andkeep the switch arm from opening and closing.

A TTD phase shifter operates on the concept of usingswitches to alter a signal path through a series of transmissionlines. Transmission lines electrical lengths directly relate tophase shift. A conventional TTD switched line phase shiftercircuit topology uses four SPST switches per bit in order toisolate each end of the transmission line from other paths. Our2-bit phase shifter has 22, or 4 total paths, and thus, 4 differentphase shifts possible. Namely, these paths give 0°, 90°, 180°,and 270° phase shifts with respect to the 0° reference path [8,9]. These phase shifters are ideally suited for phase arrayantennas. Figure 4 and Table 1 present the concept of theconventional TTD switch line phase shifter.

.......I..............I....... ------.Xapton

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90 deg

MEMS Switch phase

In

0 deg phase ref.

180 degphase

ut

Fig. 4: Circuit topology of conventional TTD Switched Line PhaseShifter

Fig. 5: Top-Down photograph of the fabricated phase shifter

Table 1: Bias scheme for the conventional TTD Switched Line PhaseShifter in Figure 4.

Phase (deg)090180270

Ii'

A0v90 V0V90 VIL

BOVOV90 V90 V

I='

C90 VOV90 VOV==

D90 V90 VOVOV

We have designed a 2-bit phase shifter using Agilent'sAdvanced Design System (ADS) software. In simulation,switches are modeled with the measured S-parameters of theLCP packaged MEMS switch to accurately represent itscharacteristics. Using measured S-parameters, we have firstcalculated the inductance required to tune out capacitancesassociated with the off-state paths. This inductance isimplemented as a short high impedance transmission line onKapton. All of the junctions and interconnects with MEMSswitches are taken into consideration in simulation. The delayand reference line lengths have been designed to move theresonances out of the frequency band of interests due to poorisolation of off-state switches. Finally, the meander lines andjunctions interconnects in the layout are analyzed in SonnetSoftware. S-parameters are imported into ADS to confirmthat design operates correctly. Simulations show less than 3.9dB average insertion loss with less than ±0.33 dB variationacross phase shifts. Figure 5 provides a photograph of thefabricated phase shifter as tested.

IV. MEASURED RESULTS

S-parameter measurements are performed on a Cascadeprobe station, using GGB Picoprobes, and an Agilent E8364BPerformance Network Analyzer (PNA). A Line-Reflect-Match (LRM) calibration establishes the reference plane to beat the probe tips. Phase shifts are measured to be accurate tospecifications at 10 GHz to within 3° phase variation.Average insertion loss is 3.9 dB with ±0.4 dB variation.Furthermore, the return loss is greater than 11 dB across thefrequency range. This shows excellent correspondence tosimulation. The excellent return loss is possible due tonegligible parasitics in the multi-layer package andinterconnects. Plots of the insertion loss per bit and return lossare provided below in figure 6, and phase shifts are plottedbelow in figure 7.

-1.5

-2

-2.5

Q -3

n-rl3.5C04

-4

-4.5

-5

Loss vs. Frequency

9 9.5 10

Frequency [GHz]10.5

0

-5

-10

cn

-15 -a

Oa

-20

-25

-301 1

Fig. 6: Measured Insertion and Return Losses

I I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

-

274

Phase Shift vs. FrequencyU

-60 90 Degrees

-120__

180 Degrees

-180

270 Degrees-240 ___ _.

-300 -MeasuredSimulated

9.5 10Frequency [GHz]

10.59 1 1

Fig. 7: Plot overlay of Measured and simulated Phase Shifts

V. CONCLUSION

This paper demonstrates the development of a multi-layerorganic module that integrates RF MEMS switches inhermetic cavities. Using this module technology we havedeveloped a fully integrated phase shifter in a system-in-a-package platform. The interconnect structures that allowelectrical connection to the switch show less than 0.1 dB. Thephase shifter achieves an insertion loss of 1.8 dB/bit with lessthan 3° of phase variation.

ACKNOWLEDGEMENTS

This work is funded in part by the NSF CAREER award,UC MICRO, General Electric, and Lockheed Martin. Wewould like to thank Oliver Boomhower at GE GlobalResearch for performing the layout of the phase shifter.

IEEE International Conference on Micro Electro MechanicalSystems, Page(s):415 - 418, Jan. 2002.

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